For many applications, powder metallurgy may provide a desirable alternative to conventionally manufactured ferrous and non-ferrous parts. The powder metallurgical process involves mixing elemental or alloy powders, compacting the mixture in a die, and sintering the resulting parts to metallurgically bond the powder metal particles. Thus, various parts, including those with complex profiles, may be fabricated without the costs associated with machining processes.
As the applications for powder metal parts grows, however, powder metal parts are increasingly being employed in tribologically stressed environments (e.g., environments that may include friction, wear, scuffing, or other types of physical stress). Powder metal parts, however, possess relatively low wear and scuffing resistance, which can result in premature failures, particularly in many sliding applications.
Powder metallurgy may be used to make components such as, for example, thrust buttons and thrust washers, which may experience tribologically stressed environments. These components may provide bearing surfaces in a variety of applications including rotary and sliding applications, and impinging force applications, and any other suitable applications. Thrust washers and thrust buttons can be included in the drive train of a machine to minimize or prevent wear due to scuffing and frictional sliding of various components of a drive train system (e.g., planet carriers, planet pads, sun gears, internal gears, side gears, drive shafts, etc.). Because of the relatively low wear and scuffing resistance of powder metal thrust buttons and thrust washers, these components may require frequent replacement. Thus, there is a need to increase the wear and scuffing resistance of powder metal parts to extend the service life of components including powder metal thrust washers and thrust buttons, among others.
At least one process for improving the wear resistance of powder metal parts has been proposed. For example, as described in ASM Handbook: Powder Metal Technologies and Applications (vol 7), powder metal parts may be subjected to a steam oxidation process for improving wear resistance of those parts. According to the described process, the powder metal components are heated in a steam atmosphere at temperatures between 510° C. and 570° C. to form an oxide surface layer. This layer may be significantly harder than the powder metal base material and may serve to fill any surface porosity of the component. As noted in the ASM Handbook, however, adhesion of the surface layer to the underlying powder metal base material is strongly influenced by the process time and temperature used in the steam oxidation technique. At process temperatures above 570° C., spalling or flaking of the surface oxide layer can occur. Further, the ASM Handbook indicates that, to avoid flaking of the surface layer due to surface tensile stress, the maximum thickness of the surface oxide layer should not exceed 7 microns.
While the described steam oxidation process disclosed in the ASM Handbook may increase the wear and scuffing resistance of certain powder metal parts, the surface oxide layer formed by the described process may be inadequate for many applications. For example, many parts, such as thrust buttons and thrust washers, for example, may be exposed to environments where the wear resistance and scuffing resistance provided by a surface oxide layer of less than 7 microns may be insufficient. As a result, the steam oxidation process described in the ASM Handbook may not extend the service life of certain powder metal parts by an appreciable amount.
The present disclosure is directed to overcoming one or more of the problems of the prior art steam oxidation technique.